The present invention relates to a polypeptide which is involved in the regulation of phosphate metabolism. More specifically, the present invention relates to a novel polypeptide Metastatic-tumor Excreted Phosphaturic-Element (MEPE) or xe2x80x9cphosphatoninxe2x80x9d. This invention also relates to genes and polynucleotides encoding phosphatonin polypeptides, as well as vectors, host cells, antibodies directed to phosphatonin polypeptides, and the recombinant methods for producing the same. Also provided are diagnostic methods for detecting disorders relating to phosphate metabolism, and therapeutic methods for treating such disorders. The invention further relates to screening methods for identifying agonists and antagonists of phosphatonin activity.
Several documents are cited throughout the text of this specification. Each of the documents cited herein (including any manufacturer""s specifications, instructions, etc.) are hereby incorporated herein by reference; however, there is no admission that any document cited is indeed prior art as to the present invention.
Phosphate plays a central role in many of the basic processes essential to the cell and the mineralization of bone. In particular, skeletal mineralization is dependent on the regulation of phosphate and calcium in the body and any disturbances in phosphate-calcium homeostasis can have severe repercussions on the integrity of bone. In the kidney, phosphate is lost passively into the glomerular filtrate and is actively reabsorbed via a sodium (Na+) dependent phosphate cotransporter. In the intestine, phosphate is absorbed from foods. A sodium (Na+) dependent phosphate cotransporter was found to be expressed in the intestine and recently cloned (Hilfiker, PNAS 95(24) (1998), 14564-14569). The liver, skin and kidney are involved in the conversion of vitamin D3 to its active metabolite, calcitriol, which plays an active role in the maintenance of phosphate balance and bone mineralization.
Vitamin D deficiency causes rickets in children and osteomalacia in adults. Both conditions are characterized by failure of calcification of osteoid, which is the matrix of bone. There are also several non-dietary conditions which can lead to rickets, including X-linked vitamin D resistant hypophosphatemic rickets (HYP), hereditary hypercalciuria with hypophosphatemic rickets (HHRH), Dent""s disease including certain types of renal Fanconi syndrome, renal I alpha-hydroxylase deficiency (VDDR 1), defects in 1,25-dihydroxy vitamin D3 receptor (end organ resistance, VDDR II), and oncogenic hypophosphatemic osteomalacia (OHO). Thus, a number of familial diseases have been characterized that result in disorders of phosphate uptake, vitamin D metabolism and bone mineralization. Recently a gene has been cloned and characterized that is defective in patients with X-linked hypophosphatemic rickets (PHEX) (Francis, Nat. Genet. 11 (1995), 130-136; Rowe, Hum. Genet. 97 (1996), 345-352; Rowe, Hum. Mol. Genet. 6 (1997), 539-549). The PHEX gene is a type II glycoprotein and a member of a family (M13), of Zn metalloendopeptidases. PHEX is proposed to function by processing a factor that plays a role in phosphate homeostasis and skeletal mineralization (Rowe, Exp. Nephrol. 5 (1997), 355-363; Rowe, Current Opinion in Nephrology and Hypertension 7(4) (1998), 367-376). Oncogenic hypophosphatemic osteomalacia (OHO), has many similarities to HYP with an overlapping pathophysiology, but different primary defects (Rowe, Exp. Nephrol. 5 (1997), 355-363; Rowe, Current Opinion in Nephrology and Hypertension 7(4) (1998), 367-376; Drezner in Primer on Metabolic Bone Diseases and Disorders of Mineral Metabolism (ed. Favus, M. J.) 184-188 (Am. Soc. Bone and Min. Res., Kelseyville, Calif., 1990)). Osteomalacia is the adult equivalent of rickets, and a key feature of tumour-acquired osteomalacia is softening of the bones. The softened bones become distorted, resulting in bow-legs and other associated changes reminiscent of familial rickets. Low serum phosphate, and abnormal vitamin D metabolism are also key features shared with HYP. Tumour acquired osteomalacia is rare, and the tumours are mainly of mesenchyrnal origin, although a number of different tumour types have also been reported (Rowe, Exp. Nephrol. 5 (1997), 355-363; Francis, Baillieres Clinical Endocrinology and metabolism 11 (1997), 145-163; loakimidis, The J. Rheumatology 21(6) (1994), 1162-1164; Lyles, Ann. Intem. Med. 93 (1980), 275-278; Rowe, Hum. Genet. 94 (1994), 457-467; Shane, Journal of Bone and Mineral Research 12 (1997), 1502-1511; Weidner, Cancer 59 (1987), 1442-1442). Surgical removal of the tumour(s) when possible, results in the disappearance of disease symptoms and bone healing, suggesting the role of a circulating phosphaturic factor(s) in the pathogenesis of the disease. Also, hetero-transplantation of tumours into nude mice (Miyauchi, J. Clin. Endocrinol. Metab. 67 (1988), 46-53) infusion of saline extracts into rats and dogs (Aschinberg, J. Paediatr. 91 (1977), 56-60; Popovtzer, Clin. Res. 29 (1981), 418A (Abstract)), and the use of tumour conditioned medium (TCM), of human and animal renal cell lines all confirm that a circulating phosphaturic factor is secreted by these tumours.
Although the primary-defect in X-linked rickets is confirmed as a mutated Zn metalloendopeptidase (PHEX), there is considerable evidence that implicates a circulating phosphaturic factor(s) (Ecarot, J. Bone Miner. Res. 7 (1992), 215-220; Ecarot, J. Bone Miner. Res. 10 (1995), 424-431; Morgan, Arch. Intern. Med. 134 (1974), 549-552; Nesbitt, J. Clin. Invest. 89 (1992), 1453-1459; Nesbitt, J. Bone. Miner. Res. 10 (1995), 1327-1333; Nesbitt, Endocrinology 137 (1996), 943-948; Qiu, Genet. Res., Camb. 62 (1993), 39-43; Lajeunesse, Kidney Int. 50 (1996), 1531-1538; Meyer, J. Bone. Miner. Res. 4(4) (1989), 523-532; Meyer, J. Bone. Miner. Res. 4 (1989), 493-500). The overlapping pathophysiology of HYP and OHO raises the intriguing possibility that the tumour-factor may be processed in normal subjects by the PHEX gene product. Also, it is likely that proteolytic processing by PHEX may act by either degrading this undefined phosphaturic factor(s), or by activating a phosphate-conserving cascade (Carpenter, Pediatric Clinics of North America 44 (1997), 443-466; Econs, Am. J. Physiol. 273 (1997), F489-F498; Glorieux, Arch. Pediatr. 4 (1997), 102s-105s; Grieff, Current Opinion in Nephrology and Hypertension 6 (1997), 15-19; Hanna, Current Therapy in Endocrinology and Metabolism 6 (1997), 533-540; Kumar, Nephrol. Dial. Transplant. 12 (1997), 11-13; Takeda, Ryoikibetsu Shokogun Shirizu (1997), 656-659). The cloning and characterization of the tumour-phosphaturic factor is thus prerequisite to establishing any links between tumour osteomalacia and familial X-linked rickets as well as other disorders in the phosphate metabolism.
Rowe et al (1996) have reported candidates 56 and 58 kDa protein (s) responsible for mediating renal defects in OHO (Rowe, Bone 18, (1996), 159-169). A patient with OHO was treated by tumor removal and pre- and post-operative antisera from the patient were used in a Western blotting identification of tumor conditioned media proteins. Neither the tumor cells nor the antisera were ever made available to the public, however.
In a review in Exp. Nephrol. 5 (1997), 335-363, Rowe (1997) discusses the above diseases and the role of the PHEX gene (previously known as the PEX gene). The PHEX gene product has been identified as a zinc metalloproteinase. In disease states such as familial rickets, defective PHEX results in uncleaved phosphatonin which would result in down regulation of the sodium dependent phosphate cotransporter and up regulation of renal mitochondrial 24-hydroxylase. However, no purification of phosphatonin was reported by Rowe (1997). Thus, no source material for phosphatonin was made available to the public. Moreover, purification, identification and characterization of phosphatonin has not been possible.
Thus, there is a need for polypeptides that regulate phosphate metabolism, since disturbances of such a regulation may be involved in hypo- and hyperphosphatemic diseases, including osteomalacia, particularly osteoporosis and renal failure. Furthermore, there is a need for identifying and characterizing such polypeptides which may play a role in the detection, prevention and/or correction of such disorders and may be useful in diagnosing those disorders.
The present invention relates to novel phosphatonin polypeptides and the encoding polynucleotides of phosphatonin. Moreover, the present invention relates to vectors, host cells, antibodies, and recombinant methods for producing the polypeptides and polynucleotides. Also provided are diagnostic methods for detecting disorders related to the polypeptides, and therapeutic methods for treating such disorders. The present invention further relates to screening methods for identifying binding partners of phosphatonin.